The present invention relates to a fuel injection device for a diesel engine or the like and, more particularly, to an improved fuel injection device which is capable of automatically controlling the injection rate in high speed and low speed engine operation ranges.
The current trend in the art of engines such as diesel engines is to the injection of fuel at higher pressures, that is, to the use of higher injection rates, which cut down smoke and hydrocarbons (HC) emissions in the low speed engine operation range. However, just as it solves the problem concerning the smoke and HC emissions, it creates another problem in the aspect of durability and nitrogen oxides (NOx) emission. Pressurizing the fuel to a substantial level in the low speed operation range would elevate the injection pressure more than necessary in the high speed operation range and require a larger drive torque for an injection pump, detrimenting the pump durability and increasing the NOx concentration in the engine exhaust.
One approach heretofore proposed to keep the fuel pressure in an injection pipe (pipe pressure) below a reference value consists in installing a relief valve in an injection pipe through which the injection pipe may be vented to the outside, as disclosed in Japanese Patent Laid-Open Publication No. 57-129250, for example. Such an approach, however, suffers from the drawback that, due to such a length of the injection pipe which communicates the pump to the nozzle, the volume and fluid resistance in the injection pipe between the relief valve and the nozzle hole are increased and effect the relief valve to lower the control accuracy and thereby render the injection characteristics unstable.